Nuclear reactor

ABSTRACT

A nuclear reactor includes a reflector and a flow path. The reflector reflects neutrons, contains graphite and a moderator having a smaller moderating power than the graphite, and is sectioned into plural parts along a direction of flow of fuel pebbles. The flow path is surrounded by the reflector, and the fuel pebbles flow through the flow path and undergo nuclear reaction to generate power. Volume ratio of the graphite to the moderator having a smaller moderating power than the graphite in each part of the reflector is determined based on a power distribution in the reactor core in the direction of flow of the fuel pebbles.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.13/115,517, filed on May 25, 2011, which is a divisional of U.S.application Ser. No. 11/965,378, filed on Dec. 27, 2007 and issued asU.S. Pat. No. 7,978,807 on Jul. 12, 2011, which is based on and claimsthe benefit of priority from Japanese Patent Application No.2007-200419, filed Aug. 1, 2007, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nuclear reactor, and moreparticularly to a nuclear reactor that allows for equalization and/oroptimization of power distribution in a reactor core.

2. Description of the Related Art

Pebble bed reactors (PBR) have a flow path (fuel region) that issurrounded by a reflector(s). Sphere-shaped fuel bodies (fuel pebbles)containing nuclear fuel material undergo nuclear reaction inside theflow path thereby producing power. In view of safety, it is preferablefor the nuclear reactors in general to have equalized power distributionin a reactor core. In the PBR, the fuel body goes down the flow path andis repeatedly removed and reloaded, whereby the power distribution inthe nuclear reactor is equalized in an axial direction (i.e., directionin which the fuel bodies flow in the flow path). Hence, while thenuclear reactor is running, replacement (removal and reloading) of thefuel bodies is continuously performed, i.e., removed fuel bodies areinspected, and new fuel bodies are loaded.

For example, a 170,000 kilowatts (kW) class nuclear reactor uses 415,000fuel bodies, among which 6,000 are removed from the nuclear reactor forinspection every day. Among the removed 6,000 fuel bodies, approximately600 are replaced with new ones. New fuel bodies are loaded into thenuclear reactor together with the approximately 5,400 remaining fuelbodies. Generally, average life of the fuel body is about two years, andone fuel body is reloaded nine times on average during its life.

However, if the replacement of the fuel bodies becomes more frequent,operational costs (costs for inspection and replenishment of the fuelbodies) increase accordingly, which in turn results in an increased costfor power generation. Hence, it is preferable to equalize the powerdistribution in the nuclear reactor without increasing the frequency offuel replacement.

One conventional technique applicable to the pebble bed reactor is knownfrom Japanese Patent Application Laid-Open No. 2003-222693. Theconventional reactor (nuclear reactor facility) described thereinincludes a detecting unit that receives plural fuel spheres dischargedfrom a reactor core and detects burnup of the fuel spheres, and asorting unit that determines a radial loading position from which thefuel sphere is reloaded into the reactor core according to the result ofdetection by the detecting unit.

In a block reactor, a fuel body having a hexagonal columnar shape housesfuel compacts. Base material of the fuel compact is graphite. The fuelcompact is filled with coated fuel particles (of approximately 1-mmdiameter). Nuclear fuel material contained in the fuel particleundergoes nuclear reaction thereby producing power. For the equalizationor the optimization of the power distribution in an axial direction,enriched uranium is prepared in twelve different levels of enrichment,for example. Preparation or manufacture of enriched uranium at variouslevels of enrichment in a large amount pushes up the manufacturing costof the fuel.

SUMMARY OF THE INVENTION

It is an object of the present invention to at least partially solve theproblems in the conventional technology. Specifically, one object of thepresent invention is to provide a nuclear reactor which allows forequalization and/or optimization of power distribution in a reactorcore.

In order to achieve an object as described above, a nuclear reactor(pebble bed reactor) according to one aspect of the present inventionincludes a reflector reflecting neutrons and containing graphite andmoderator having a smaller moderating power than the graphite, thereflector being sectioned into plural parts along a direction of flow offuel pebbles, and a flow path surrounded by the reflector, and throughthe flow path the fuel pebbles (fuel bodies) flow and undergo nuclearreaction to generate power. A volume ratio of the graphite to themoderator having a smaller moderating power than the graphite in eachpart of the reflector is determined based on a power distribution of areactor core in the direction of flow of the fuel pebbles.

Further, in the nuclear reactor (pebble bed reactor) according toanother aspect of the present invention, when a power density obtainedwith the reflector consisting of graphite is taken as a reference, apart of the reflector located at a position with a higher power densitycontains a higher volume fraction of the moderator having a smallermoderating power than the graphite than a part of the reflector locatedat a position with a lower power density.

Still further, in the nuclear reactor (pebble bed reactor) according tostill another aspect of the present invention, the reflector contains ahigher volume fraction of the moderator having a smaller moderatingpower than the graphite in a portion located at an upstream side of theflow path of the fuel pebbles than in a portion located at a downstreamside of the flow path of the fuel pebbles.

Still further, in the nuclear reactor (pebble bed reactor) according tostill another aspect of the present invention, the reflector includes aninner reflector and an outer reflector that surrounds a reactor core,the flow path of the fuel pebbles is surrounded by an outercircumference of the inner reflector and an inner circumference of theouter reflector, and a volume fraction of the moderator having a smallermoderating power than the graphite is set equal to or higher than 25% inat least an approximately ⅓ portion of the inner reflector from theupstream side of the flow path of the fuel pebbles.

Still further, in the nuclear reactor (pebble bed reactor) according tostill another aspect of the present invention, the volume fraction ofthe moderator having a smaller moderating power than the graphite is setequal to or higher than 25% in an approximately ⅔ portion of the innerreflector from the upstream side of the flow path of the fuel pebbles.

Still further, in the nuclear reactor (pebble bed reactor) according tostill another aspect of the present invention, the reflector includes aninner reflector and an outer reflector that surrounds a reactor core,the flow path of the fuel pebbles is surrounded by an outercircumference of the inner reflector and an inner circumference of theouter reflector, and the volume fraction of the moderator having asmaller moderating power than the graphite is set equal to or higherthan 75% in at least an approximately ⅓ portion of each of the innerreflector and the outer reflector from the upstream side of the flowpath of the fuel pebbles.

Still further, in the nuclear reactor (pebble bed reactor) according tostill another aspect of the present invention, the reflector includes aninner reflector and an outer reflector that surrounds a reactor core,the flow path of the fuel pebbles is surrounded by an outercircumference of the inner reflector and an inner circumference of theouter reflector, and the volume fraction of the moderator having asmaller moderating power than the graphite is set equal to or higherthan 75% in an approximately ⅓ portion of the inner reflector from theupstream side of the flow path of the fuel pebbles, equal to or higherthan 25% in an approximately ⅓ portion at the center of the innerreflector, and equal to or higher than 25% in an approximately ⅓ portionof the outer reflector from the upstream side.

Still further, in the nuclear reactor (pebble bed reactor) according tostill another aspect of the present invention, criticality of thereactor core is adjusted by enrichment of the fuel pebbles.

Still further, a nuclear reactor (block reactor) according to stillanother aspect of the present invention includes a reactor core, a fuelblock arranged in the reactor core, the fuel block having a coolant flowpath through which a coolant flows to cool the fuel block, and areflector arranged inside the reactor core and sectioned into pluralparts along a flow direction of the coolant, the reflector containinggraphite and a moderator having a smaller moderating power than thegraphite, and a volume ratio of the graphite to the moderator having asmaller moderating power than the graphite in each part of the reflectoris determined based on a power distribution in the reactor core in theflow direction of the coolant.

Still further, in the nuclear reactor (block reactor) according to stillanother aspect of the present invention, a volume fraction of themoderator having a smaller moderating power than the graphite in eachpart of the reflector is determined so that the volume fractionincreases from an upstream side to a downstream side of the coolant flowpath.

The above and other objects, features, advantages and technical andindustrial significance of this invention will be better understood byreading the following detailed description of presently preferredembodiments of the invention, when considered in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a nuclear reactor according to thefirst embodiment of the present invention;

FIG. 2 is a schematic view of an outer reflector and an inner reflectorof the nuclear reactor shown in FIG. 1;

FIG. 3 is a schematic diagram of fuel pebble employed in the nuclearreactor of FIG. 1;

FIG. 4 is a schematic diagram of a coated fuel particle employed in thenuclear reactor of FIG. 1;

FIG. 5 is a schematic diagram illustrating a general operation of thenuclear reactor shown in FIG. 1;

FIG. 6 is a table of a first set of specific examples of composition ofreflectors in the nuclear reactor of FIG. 1;

FIG. 7 is a graph illustrating axial power distribution in examples ofthe nuclear reactor shown in FIG. 6;

FIG. 8 is a table of a second set of specific examples of composition ofreflectors in the nuclear reactor of FIG. 1;

FIG. 9 is a graph illustrating power distribution in examples of thenuclear reactor shown in FIG. 8;

FIG. 10 is a perspective view of a nuclear reactor according to thesecond embodiment of the present invention;

FIG. 11 is a schematic diagram of a reactor core of the nuclear reactorshown in FIG. 10;

FIG. 12 is a perspective view of a block-type fuel block(multi-hole-type fuel body) employed in the nuclear reactor shown inFIG. 10;

FIG. 13 is a plan view of the block-type fuel block (multi-hole-typefuel body) employed in the nuclear reactor shown in FIG. 10;

FIG. 14 is a sectional view of the block-type fuel block(multi-hole-type fuel body) employed in the nuclear reactor shown inFIG. 10 along a line A-A of FIG. 13;

FIG. 15 schematically shows a reactor core of a conventional nuclearreactor;

FIG. 16 schematically shows a specific example of the reactor core ofthe nuclear reactor shown in FIG. 10;

FIG. 17 is a graph of power distribution in the nuclear reactor shown inFIG. 16;

FIG. 18 is a graph of neutron flux distribution in a radial direction inthe conventional (pebble bed) nuclear reactor; and

FIG. 19 is a graph of neutron flux distribution in an axial direction inthe conventional (pebble bed) nuclear reactor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in detail below with referenceto the accompanying drawings. It should be noted that the presentinvention is not limited by embodiments described below. Elementsincluded in the embodiments may be replaceable with or substantiallyequivalent to alternative elements easily conceived by those skilled inthe art. Further, it should be obvious to those skilled in the art thatmodifications described in relation to the embodiments below mayoptionally be combined with each other without departing from the scopeof the present invention.

First Embodiment

FIG. 1 is a schematic diagram of a nuclear reactor according to thefirst embodiment of the present invention. FIG. 2 is a schematic view ofan outer reflector and an inner reflector of the nuclear reactor shownin FIG. 1. FIG. 3 is a schematic diagram of a fuel pebble and FIG. 4 isa schematic diagram of a coated fuel particle. FIG. 5 is a schematicdiagram illustrating how the fuel bodies move within the nuclear reactorshown in FIG. 1 for fuel replacement. FIG. 6 is a table showing a firstset of specific examples of composition of the reflectors of the nuclearreactor shown in FIG. 1. FIG. 7 is a graph showing power distribution inthe examples of the nuclear reactor shown in FIG. 6. FIG. 8 is a tableshowing a second set of specific examples of composition of thereflectors of the nuclear reactor shown in FIG. 1. FIG. 9 is a graphshowing power distribution in the examples of the nuclear reactor shownin FIG. 8. FIGS. 18 and 19 are graphs of neutron flux distribution in aconventional nuclear reactor.

Pebble Bed Reactor

A nuclear reactor 1 is applied, for example, to a pebble bed reactor(see FIG. 1). The nuclear reactor 1 includes an outer reflector 2 and aninner reflector 3. The reflectors 2 and 3 surround a flow path (fuelregion) R. The nuclear reactor 1 induces nuclear reaction (nuclearfission or capture, for example) of nuclear fuel materials contained infuel pebbles (fuel bodies) 4 in the flow path R, thereby generatingpower. In the nuclear reactor 1, since the fuel pebbles 4 circulatewithin the flow path R, the power distribution is equalized in an axialdirection of a reactor core (direction of flow of the fuel pebbles 4).While the nuclear reactor 1 is running, the fuel pebbles 4 arecontinuously replaced (removed and reloaded), and accordingly aninspection of the removed fuel pebbles 4 and reloading of new fuelpebbles 4 are performed. In the nuclear reactor 1, helium gas isemployed as a coolant.

The outer reflector 2 is a cylindrical member of graphite (C) andsilicon carbide (SiC), and constitutes an outer wall of the reactor core(see FIGS. 1 and 2). The outer reflector 2 has a function of reflectingand shielding neutrons emitted from the fuel pebbles 4 at an outercircumference of the reactor core. The inner reflector 3 is acolumn-like member of graphite (C) (and silicon carbide (SiC)), and isarranged on a central axis of the outer reflector 2. The inner reflector3 has a function of reflecting the neutrons emitted from the fuelpebbles 4 at a center of the reactor core. The inner reflector 3 alsofunctions as a moderator to moderate the neutrons. The flow path R ofthe fuel pebbles 4 is between the outer reflector 2 and the innerreflector 3. The flow path R is formed of gaps between the sphericalfuel pebbles, and extends in the axial direction of the reactor core.

The fuel body (fuel pebble) 4 includes a graphite shell 41 and pluralcoated fuel particles 42 embedded in the graphite shell 41 (see FIG. 3).The graphite shell 41 is a sphere of approximately 3 centimeters (cm) inradius. The coated fuel particle 42 is a fuel particle of approximately0.46 millimeter (mm) in radius. Plural coated fuel particles 42 aredispersed in a central portion (a portion within a range ofapproximately 2.5 cm in radius) of the graphite shell 41. One graphiteshell 41 contains many coated fuel particles 42. The coated fuelparticle 42 includes a fuel kernel 421 of approximately 0.25 mm inradius, a gap 422, a low-density pyrolytic carbon (PyC) buffer layer423, a sealing layer 424, a high-density pyrolytic carbon (PyC) innerlayer 425 of approximately 0.040 mm in thickness in radial direction, asilicon carbide layer 426 of approximately 0.035 mm in thickness inradial direction, and a high-density pyrolytic carbon (PyC) outer layer427 of approximately 0.040 mm in thickness in radial direction (see FIG.4). The gap 422, the low-density pyrolytic carbon (PyC) buffer layer423, and the sealing layer 424 together form a layer of approximately0.095 mm in thickness in radial direction. The coated fuel particle 42has the fuel kernel 421 as a core and the layers 421 to 426 stacked inthe above mentioned order thereon. The fuel kernel 421 is uranium-235(²³⁵U) enriched to approximately 8 wt %.

While the nuclear reactor 1 is running, the fuel pebbles 4 arecontinuously loaded to the reactor core from an upper side of thenuclear reactor 1 (see FIG. 5). Then, the fuel pebbles 4, while flowingthrough the flow path R inside the reactor core, undergo nuclearreaction, thereby generating thermal energy. After passing through theflow path R, the fuel pebbles 4 are removed from the nuclear reactor 1from a lower side thereof. Then, a predetermined inspection is carriedout on the removed fuel pebbles 4 to check burnup, condition, or thelike. If reloadable, the fuel pebble 4 is reloaded to the reactor corefrom the upper side of the nuclear reactor 1. If the burnup has exceededa predetermined level (for example 80 GWd/t), or if the condition is notgood, the fuel pebble 4 is taken away and stored. FIG. 5 shows batchesof the fuel pebbles 4 (assembly of fuel pebbles) move from the upperside to the lower side of the nuclear reactor 1 along zones inside theflow path R. Each of arrows in FIG. 5 indicates a travel distance of thefuel pebbles 4 per unit time.

Composition of Reflector

Generally, it is preferable for safety, for example, to equalize thepower distribution (depended on distribution of neutron flux) in aradial direction and in an axial direction inside the reactor core ofthe pebble bed reactor. For example, if the nuclear reactor employs areflector consisting of graphite, a peak of power density appears in theneighborhood of a graphite region due to neutron flux distribution inthe radial direction in the reactor core as shown in FIG. 18. If thedistribution of neutron flux is taken along the axial direction of thereactor core (flow direction of the fuel pebbles 4), the peak appears ata center of the reactor core (see FIG. 19). The peaks appear asdescribed above because uranium-235 contained in the fuel pebble 4 tendsto undergo fission reaction more actively on colliding againstsoft-spectrum neutron flux than hard-spectrum neutron flux, and alsobecause that the reflector consisting of graphite makes the neutron fluxspectrum soft (i.e., thermalized).

Generally, when the reflector contains a high volume fraction of siliconcarbide and a low volume fraction of graphite (SiC/C reflector), theneutrons are hardly moderated and hence spread afar. Contrarily, whenthe reflector contains a high volume fraction of graphite (C reflector,or SiC/C reflector), the neutrons are well moderated and easilythermalized. The thermalized neutrons (thermal neutrons) react well withthe nuclear fuel material to easily undergo fission reaction.

In the nuclear reactor 1, the outer reflector 2 and the inner reflector3 (or at least the inner reflector 3) contain graphite and a moderatorwhich has a smaller moderating power than the graphite (silicon carbide,for example), and the reflectors 2 and 3 (or at least 3) are sectionedinto plural parts in the direction of the flow path R of the fuelpebbles 4. Further, the volume ratio of the graphite to the moderatorhaving a smaller moderating power than the graphite in each part of thereflectors 2 and 3 is determined based on a power distribution in thereactor core in the direction of the flow path R of the fuel pebbles 4(see FIG. 2). For example, the inner reflector 3 is sectioned into anupper part 31, a central part 32, and a lower part 33 in the directionof flow path R of the fuel pebbles 4, and the volume ratio of thegraphite to the moderator having a smaller moderating power than thegraphite in each of the parts 31 to 33 is determined based on the powerdistribution in the reactor core (see FIGS. 6 to 9).

According to the above described arrangement, the volume fractions ofelements in the reflectors 2 and 3 (in particular, the volume fractionof the moderator having a smaller moderating power than the graphite)are determined based on the power distribution in the reactor core, sothat a degree of moderation of the neutrons at each position in the flowpath R of the fuel pebbles 4 is adjusted. For example, at a positionwhere the volume fraction of the moderator having a smaller moderatingpower than the graphite is set high, the degree of moderation of theneutrons is low, whereby the power density is suppressed. Contrarily, ata position where the volume fraction of the moderator having a smallermoderating power than the graphite is set low, the degree of moderationof the neutrons is high, whereby the power density is enhanced. Thus,the arrangement as described above is advantageous in that the powerdistribution in the reactor core can be adjusted, and that the powerdistribution in the reactor core can be equalized.

Further, when the power distribution in the reactor core is equalized,the number of replacements (the number of reloads and removals) of thefuel pebbles 4, performed for equalizing the composition of the reactorcore in the axial direction, can be reduced. Thus, the above describedarrangement is advantageous in that the operational costs accompanyingthe replacement of the fuel pebbles 4 (costs of inspection andreplenishment of the fuel pebbles 4) can be reduced. Further, the abovedescribed arrangement is advantageous in that the cost of powergeneration can be reduced, since the above described arrangement allowsfor an economical operation of the nuclear reactor 1. Still further, theabove described arrangement is advantageous in that the reliability ofthe nuclear reactor 1 can be enhanced since the number of reloads of thefuel pebbles 4 is decreased, i.e., since there is less possibility ofentrainment of impurity into the fuel, which may occur at the time ofreloading.

For the reflectors of the nuclear reactor 1, silicon carbide is employedas the moderator having a smaller moderating power than the graphite.The moderator, however, is not limited to silicon carbide. For example,tungsten, molybdenum, or one of carbides that have an excellent hightemperature resistance other than silicon carbide may be employed as themoderator having a smaller moderating power than graphite. Similarly,low density graphite may be employed.

If the power density in the nuclear reactor which employs the reflectors2 and 3 that contain graphite alone is set as a standard, for example,the reflectors 2 and 3 are preferably formed so that portions located inpositions with high power density contain a higher volume fraction ofsilicon carbide (i.e., the moderator having a low moderating power thangraphite) than portions located in positions with low power density. Inother words, at a position where it is desirable to suppress the maximumpower density of the reactor core (i.e., where the power density ishigh), the volume fraction of silicon carbide in each of the reflectors2 and 3 is set high (i.e., the volume fraction of graphite is set low).Contrarily, the volume fraction of graphite in each of the reflectors 2and 3 is set high (i.e., the volume fraction of silicon carbide is setlow, or to zero) at positions where it is desirable to increase thepower density (i.e., where the power density is low). Then, the neutronspectrum (distribution of neutron energy) shifts towards a high side ofenergy. Then, the nuclear reaction becomes difficult to occur, wherebythe output of the portion decreases. Thus, the power distribution in thereactor core can be adjusted, and the power distribution in the reactorcore in the direction of flow of the fuel pebbles 4 can be effectivelyequalized.

Further, in the pebble bed reactor 1, new fuel pebbles 4 are loaded fromthe upper side of the reactor core. Therefore, the power density tendsto be higher at the upstream side of the flow path R than at thedownstream side of the flow path R. Hence, in the reflectors 2 and 3, itis preferable to set the volume fraction of silicon carbide higher inportions located at the upstream side of the flow path R than inportions located at the downstream side of the flow path R. With sucharrangement, the degree of moderation of neutrons at the upstream sideof the flow path R is decreased, to suppress the maximum power densityof the reactor core. Then, the power density is increased at thedownstream side of the flow path R owing to the suppressed amount at theupstream side under normal power. Thus, the nuclear reactor 1 isadvantageous in that the power distribution in the reactor core in theflow direction of the fuel pebbles 4 can effectively be equalized.

First Set of Specific Examples of Reflectors

When the nuclear reactor 1 includes the reflectors 2 and 3, i.e., theouter reflector 2 and the inner reflector 3 as described above (seeFIGS. 1 and 2), it is preferable that the volume fraction of themoderator (silicon carbide) having a smaller moderating power than thegraphite be set equal to or higher than 25% at least in an approximately⅓ portion from the upstream side of the flow path R of the fuel pebbles4 (i.e., at least in the upper part 31 of the inner reflector 3; i.e.,approximately ⅓) (see FIGS. 6 and 7). When the reflectors have suchcomposition, the degree of moderation of neutrons is decreased at theupstream side of the flow path R, to suppress the maximum power density.Then, the power density increases at the downstream side of the flowpath R owing to a suppressed amount at the upstream side. Thus, thenuclear reactor 1 is advantageous in that the power distribution in thereactor core in the flow direction of the fuel pebbles 4 is effectivelyequalized.

Further, it is preferable in the nuclear reactor 1 that the volumefraction of the moderator having a smaller moderating power than thegraphite be set equal to or higher than 25% in the inner reflector 3 ina portion within the range of approximately ⅔ from the upstream side ofthe flow path R of the fuel pebbles 4 (i.e., the upper part 31 and thecentral part 32 of the inner reflector 3) (see FIGS. 6 and 7). Suchcomposition is advantageous in comparison with the above describedcomposition in that the power distribution in the reactor core in thedirection of flow of the fuel pebbles 4 is more effectively equalized.In the first embodiment, the composition of the reflectors 2 and 3 isdefined in terms of volume.

In the conventional nuclear reactor with the reflectors having thecompositions shown in FIG. 6 and the reactor core having the powerdistribution shown in FIG. 7, both the inner reflector and the outerreflector consist only of graphite.

On the other hand, in an example 1 of the nuclear reactor 1, the innerreflector 3 is trisected into the upper part 31, the central part 32,and the lower part 33 from the upstream side of the flow path R in thisorder. The volume ratio of the silicon carbide (moderator having asmaller moderating power than graphite) to the graphite in the upperpart 31 and the central part 32 in the inner reflector 3 is set to25:75. The lower part 33 of the inner reflector 3 and the outerreflector 2 consist of graphite.

In an example 2 of the nuclear reactor 1, the volume ratio of thesilicon carbide to the graphite in the upper part 31 and the centralpart 32 in the inner reflector 3 is set to 50:50. The lower part 33 ofthe inner reflector 3 and the outer reflector 2 consist of graphite.

In an example 3 of the nuclear reactor 1, the volume ratio of thesilicon carbide to the graphite in the upper part 31 and the centralpart 32 in the inner reflector 3 is set to 75:25. The lower part 33 ofthe inner reflector 3 and the outer reflector 2 consist of graphite.

In an example 4 of the nuclear reactor 1, the upper part 31 and thecentral part 32 of the inner reflector 3 consist of silicon carbide. Thelower part 33 of the inner reflector 3 and the outer reflector 2 consistof graphite.

As shown in results of calculations in FIG. 7, in the nuclear reactorsof the examples 1 to 4, the power density at the upstream side of theflow path R is suppressed in comparison with the conventional nuclearreactor, while the power density at the downstream side of the flow pathR is increased by the suppressed amount. For example, when theconventional example is compared with the example 4, the maximum powerdensity of the example 4 is suppressed to approximately ⅓ of the maximumpower density of the conventional example. The results of calculationsin FIG. 7 show relation between the positions in the axial direction inthe reactor core and the maximum power density near a position radiallyinside the reactor core.

Second Set of Specific Examples of Reflectors

Further, in the nuclear reactor 1, it is preferable that the volumefraction of the moderator having a smaller moderating power thangraphite in each of the inner reflector 3 and the outer reflector 2 beset equal to or higher than 25% in a ⅓ portion from the upstream side ofthe flow path R of the fuel pebbles 4 (see FIGS. 8 and 9). With such acomposition, the degree of moderation of neutrons decreases at theupstream side of the flow path R to suppress the power density near theupstream side. In addition, since the degree of moderation of neutronsis adjusted by both the inner reflector 3 and the outer reflector 2, thepower density can effectively be suppressed. The output at thedownstream side of the flow path R is, then increased owing to thesuppressed amount at the upstream side under normal power. Thus, theabove described composition is advantageous in that the powerdistribution in the reactor core in the flow direction of the fuelpebbles 4 can effectively be equalized.

Further, in the nuclear reactor 1, it is preferable that the volumefraction of the moderator having a smaller moderating power thangraphite in the inner reflector 3 be set equal to or higher than 25% inan approximately ⅔ portion from the upstream side of the flow path R ofthe fuel pebbles 4 (see FIGS. 8 and 9). The composition is advantageousin comparison with the above described composition in that the powerdistribution in the reactor core in the flow direction of the fuelpebbles 4 can more effectively be equalized.

In the nuclear reactor 1, the criticality is adjusted through theadjustment of enrichment (uranium enrichment) of the fuel pebbles 4.

For example, in an example 5 of the nuclear reactor 1 having thereflectors with the composition as shown in FIG. 8 and the powerdistribution as shown in FIG. 9, the inner reflector 3 is trisected intothe upper part 31, the central part 32, and the lower part 33 from theupstream side of the flow path R in this order. The outer reflector 2 isalso trisected into the upper part 21, the central part 22, and thelower part 23, in this order from the upstream side of the flow path R.The volume ratio of the silicon carbide (moderator having a smallermoderating power than the graphite) to the graphite in each of the upperpart 31 of the inner reflector 3 and the upper part 21 of the outerreflector 2 is set to 75:25. The central part 32 and the lower part 33of the inner reflector 3 and the central part 22 and the lower part 23of the outer reflector 2 consist of graphite.

In an example 6 of the nuclear reactor 1, the upper part 31 of the innerreflector 3 consists of the moderator having a smaller moderating powerthan graphite. Further, the volume ratio of the silicon carbide to thegraphite in the upper part 21 of the outer reflector 2 is set to 75:25.Further, the central part 32 and the lower part 33 of the innerreflector 3 and the central part 22 and the lower part 23 of the outerreflector 2 consist of graphite.

In an example 7 of the nuclear reactor 1, the volume ratio of thesilicon carbide to the graphite in each of the upper part 31 of theinner reflector 3 and the upper part 21 of the outer reflector 2 is setto 75:25. Further, the volume ratio of the silicon carbide to thegraphite in the central part 32 of the inner reflector 3 is set to25:75. Further, the lower part 33 of the inner reflector 3 and thecentral part 22 and the lower part 23 of the outer reflector 2 consistof graphite.

In an example 8 of the nuclear reactor 1 which has the same compositionas the example 7 of the nuclear reactor 1, the criticality is maintainedthrough the adjustment of uranium enrichment in the fuel pebbles 4.

As shown in the results of calculations of FIG. 9, the power density atthe upstream side of the flow path R is suppressed while the powerdensity at the downstream side of the flow path R is increased owing tothe suppressed amount at the upstream side under normal power in thenuclear reactors of examples 5 to 8 in comparison with the conventionalnuclear reactor. For example, the maximum power density of the example 9is suppressed to a level equal to or lower than approximately ⅓ themaximum power density of the conventional nuclear reactor. The powerdistribution shown in FIG. 9 illustrates relation between the positionin the axial direction of the reactor core and the maximum power densitynear a position radially inside the reactor core.

Second Embodiment

FIG. 10 shows a schematic structure of a nuclear reactor according tothe second embodiment of the present invention. FIG. 11 is anexplanatory view of a reactor core of the nuclear reactor shown in FIG.10. FIGS. 12 to 14 are a perspective view (FIG. 12), a plan view (FIG.13), and a sectional view along line A-A of FIG. 13 (FIG. 14) of a fuelblock (fuel body) employed in the nuclear reactor of FIG. 10,respectively. FIG. 15 shows a schematic structure of the reactor core ofthe conventional nuclear reactor. FIG. 16 shows a schematic structure ofa specific example of a reactor core of the nuclear reactor shown inFIG. 10. FIG. 17 is a graph showing results of power distribution of thenuclear reactor shown in FIG. 16.

Block-Type High Temperature Gas Reactor

A nuclear reactor 10 is, for example, applied to a block-type hightemperature gas reactor (see FIG. 10). The nuclear reactor 10 includes asteel nuclear reactor vessel 11, a reactor core 12, reactor internals 13that support the reactor core 12, and a piping 14 for a cooling system.Coolant flows through the piping 14. In the nuclear reactor 10, thereactor core 12 and the reactor internals 13 are housed inside thenuclear reactor vessel 11. Helium gas is employed as the coolant, forexample.

The reactor core 12 includes outer reflectors 121, fuel blocks (fuelbodies) 122, control rod guide columns 123, irradiation test columns124, inner reflectors (graphite reflector columns) 125, and the like(see FIGS. 11 and 12). The outer reflector 121 is made of graphite in ablock-shape. Graphite blocks are arranged in a circle to form an outerperipheral portion of the reactor core 12. The fuel block 122, thecontrol rod guide column 123, the irradiation test column 124, and theinner reflector 125 are each hexagonal column-shaped, and arrangedinside the outer reflector 121 with their longitudinal directions (axialdirections) in line with the axial direction of the reactor core 12. Inthe nuclear reactor 10, the reactor core 12 is sectioned into fourregions in the radial direction, and the fuel loading is performedindependently for each region (see FIG. 11). Thus, the distribution offuel temperature is optimized, and the maximum fuel temperature isminimized.

The fuel block 122 includes a hexagonal columnar graphite block 1221,fuel rods 1222 that are inserted into the graphite block 1221 and heldthereby, burnable poison rods 1223, and coolant flow paths 1225 (seeFIG. 13). Further, the fuel block 122 is formed with a cylindricalgraphite sleeve and a fuel compact sealed therein. The fuel compact hasa cylindrical shape. In the fuel compact, plural coated fuel particlesare dispersed in a graphite base material (graphite substrate). Further,the coated fuel particle includes a fuel kernel of uranium dioxide (UO₂)and three thin layers of pyrolytic carbon (PyC) or silicon carbide (SiC)formed on the fuel kernel.

In the fuel block 122, plural coolant flow paths 1225 run in thelongitudinal direction of the fuel block 122 (see FIG. 14). Through thecoolant flow path 1225, the coolant (helium gas) flows from the upperside of the reactor core 12 to the lower side of the reactor core 12.FIG. 14 shows the flow direction of the coolant by an arrow. Aftercooling the fuel block 122, the coolant flows through the piping 14 andis recovered outside the nuclear reactor 11 (see FIG. 10).

Conventional Block-Type Reactor Core

Generally, in the block-type high temperature gas reactor, it ispreferable for safety, for example, that the power distribution(depended on distribution of neutron flux) in the axial direction in thereactor core be optimized to reduce the maximum fuel temperature. In theconventional nuclear reactor, fuel is employed at various degrees ofuranium enrichment in the reactor core (in the example described herein,fuel is employed at twelve different degrees of uranium enrichment) forthe optimization of the power distribution in the axial direction (seeFIG. 15).

In such an arrangement, the enrichment of the nuclear fuel in eachportion of the reactor core 12 is determined based on the powerdistribution in the reactor core, whereby the nuclear reaction of thenuclear fuel material in the fuel blocks 122 at each position of thereactor core 12 is adjusted. For example, at a position where theenrichment of the nuclear fuel is set high, the nuclear reaction isaccelerated to enhance the power density. Contrarily, at a positionwhere the enrichment of the nuclear fuel is set low, the nuclearreaction is decelerated to suppress the power density. Thus, the outputof the reactor core 12 can be adjusted. Such an arrangement isadvantageous in that the power distribution in the reactor core 12 canbe optimized.

Specifically, the enrichment of the nuclear fuel in each portion of thereactor core 12 is determined so that the enrichment lowers from theupstream side of the coolant flow path 1225 towards the downstream sidethereof (i.e., from the upper side to the lower side of the reactor core12) (see FIG. 15). In other words, the enrichment of the nuclear fuel isset high at the upstream side of the reactor core 12 (upper side of thereactor core 12) to enhance the power density, whereas the enrichment ofthe nuclear fuel is set low at the downstream side of the reactor core12 (lower side of the reactor core 12) to suppress the power density.Thus, the power distribution in the reactor core in the flow directionof the coolant is effectively optimized and the maximum fuel temperatureis suppressed to a low level. In FIG. 15, numerical values shown insidethe portions of fuel blocks 122 indicate the enrichment of the nuclearfuel.

In the above structure, it is preferable that the inner reflector(graphite reflector column) 125 that reflects the neutrons be arrangedcloser to the center of the reactor core 12 in the radial direction thanthe fuel blocks 122 in the reactor core 12 (see FIG. 15). In suchstructure, the inner reflector 125 arranged closer to the center of thereactor core 12 in the radial direction reflects and moderates theneutrons. Thus, the structure is advantageous in that the neutrons areeffectively moderated, and that the maximum fuel temperature which isreached at the center of the reactor core at the depressurizationaccident, for example, can be suppressed to a low level. In FIG. 15,numerical values shown inside the portions of fuel blocks 122 indicatethe uranium enrichment of the nuclear fuel.

For example, in the example shown in FIGS. 15 and 16, in theconventional nuclear reactor, the enrichment of the nuclear fuel in eachportion of the reactor core 12 is set so that the enrichment decreasesfrom the upstream side toward the downstream side of the reactor core 12(i.e., from the upper side toward the lower side of the reactor core12). At the same time, the inner reflector 125 is arranged inside thereactor core 12 closer to the center of the reactor core 12 in theradial direction than the fuel blocks 122.

Specific Example of Reactor Core

In the nuclear reactor 10, it is preferable that the reflector 126 whichcontains graphite and moderator that has a smaller moderating power thanthe graphite (for example, silicon carbide) be arranged inside thereactor core 12; that the reflector 126 is sectioned into plural partsalong the flow direction of the coolant; and that the volume ratio ofthe graphite to the moderator having a smaller moderating power than thegraphite in each part of the reflector 126 is determined based on thepower distribution in the reactor core 12 in the flow direction of thecoolant (see FIG. 16).

In such structure, since the volume ratio of the graphite to the siliconcarbide (moderator having a smaller moderating power than the graphite)in each portion of the reflector 126 is determined based on the powerdistribution in the reactor core, the power density at each position inthe reactor core 12 is adjusted. For example, at a position where thevolume fraction of the silicon carbide is set high, the degree ofmoderation of neutrons becomes low, whereby the power density issuppressed. Contrarily, at a position where the volume fraction ofsilicon carbide is set low, the moderation of neutrons is accelerated toenhance the power density. Thus, the structure is advantageous in thatthe power distribution in the reactor core 12 can be adjusted, and thatthe power distribution in the reactor core 12 can be optimized.

Further, it is preferable that the volume ratio of the graphite to themoderator having a smaller moderating power than the graphite inrespective portions of the reflector 126 in the nuclear reactor 10 bedetermined so that the volume ratio increases from the upstream sidetoward the downstream side of the reactor core 12 (from the upper sidetoward the lower side of the reactor core 12) (see FIG. 16). In otherwords, the volume fraction of the graphite is set high at the upstreamside of the reactor core 12 (upper side of the reactor core 12) toenhance the output of the reactor core 12. Contrarily, the volumefraction of the moderator having a smaller moderating power than thegraphite is set high at the downstream side of the reactor core 12(lower side of the reactor core 12) to suppress the power density. Thus,the structure is advantageous in that the power distribution in thereactor core 12 can be adjusted, and that the power distribution in thereactor core 12 in the flow direction of the coolant can effectively beoptimized.

For example, the inner reflector 126 in an example 11 shown in FIG. 16is sectioned into five parts along the flow direction of the coolant(axial direction of the reactor core 12), so that the volume fraction ofsilicon carbide increases by 25% in each part from the upper side towardthe lower side of the reactor core 12. The reflector 126 is arranged inthe second layer and the fifth layer from the center of the reactor core12 in the radial direction. Thus, the plural reflectors 126 are arrangedso as to sandwich the fuel blocks 122 from the inner side and the outerside in the radial direction of the reactor core 12, and so as to belocated near the fuel blocks 122.

The numerical values shown in the fuel blocks 122 in FIG. 16 indicatethe enrichment of the nuclear fuel, whereas the numerical values in theinner reflectors 126 indicate the volume fraction of the moderator(silicon carbide) having a smaller moderating power than the graphite.For example, if the numerical value in the reflector 126 is 25%, itmeans that the volume ratio of the moderator having a smaller moderatingpower than the graphite to the graphite is 25:75. In the secondembodiment, the composition of the inner reflector 126 is defined byvolume.

As shown by the results of calculations shown in FIG. 17, it can be seenthat the power distribution in the reactor core 12 in the flow directionof the coolant in the nuclear reactor 10 of the example 11 is furtheroptimized in comparison with the power distribution in the conventionalexample.

As can be seen from the foregoing, in the nuclear reactor (pebble bedreactor) according to one aspect of the present invention, compositionof the reflector (in particular, the volume fraction of the moderatorwhich has a smaller moderating power than graphite) is determined basedon the power distribution in the reactor core. A degree of moderation ofneutrons at each position in the flow path of the fuel pebbles isadjusted, accordingly. Thus, the nuclear reactor according to one aspectof the present invention is advantageous in that the power distributionin the reactor core can be adjusted, and that the power distribution inthe reactor core can be equalized accordingly.

In the nuclear reactor (pebble bed reactor) of another aspect of thepresent invention, the volume fraction of the moderator, which has asmaller moderating power than the graphite, in a portion of thereflector is set high, when the portion is located in a region with highpower density. Therefore, the neutron spectrum (neutron energydistribution) shifts towards a side of a higher energy level, tosuppress the nuclear reaction, whereby the power density of the portionis decreased. Thus, the nuclear reactor according to another aspect ofthe present invention is advantageous in that the power distribution inthe reactor core is effectively equalized in the flow direction of thefuel pebbles.

In the nuclear reactor (pebble bed reactor) according to still anotheraspect of the present invention, the degree of moderation of neutrons isdecreased at the upstream side of the flow path to suppress the powerdensity. Under a condition set to keep the total power of the reactorcore constant, the power density at a downstream side of the flow pathincreases owing to the suppressed amount of output at the upstream side.Thus, the nuclear reactor according to the still another aspect of thepresent invention is advantageous in that the power distribution in thereactor core is effectively equalized in the flow direction of the fuelpebbles.

In the nuclear reactor (pebble bed reactor) according to still anotheraspect of the present invention, the degree of moderation of neutrons atthe upstream side of the flow path is decreased to suppress the powerdensity. Under the condition set to keep the total power of the reactorcore constant, the power density at the downstream side of the flow pathis increased owing to the suppressed amount of output at the upstreamside. Thus, the nuclear reactor according to the still another aspect ofthe present invention is advantageous in that the power distribution inthe reactor core is effectively equalized in the flow direction of thefuel pebbles.

The nuclear reactor (pebble bed reactor) according to the still anotheraspect of the present invention is advantageous in comparison with thenuclear reactor according to the other aspects in that the powerdistribution in the reactor core is more effectively equalized in theflow direction of the fuel pebbles.

In the nuclear reactor (pebble bed reactor) according to still anotheraspect of the present invention, the degree of moderation of neutrons isdecreased at the upstream side of the flow path, to suppress the powerdensity. Further, since the neutrons are moderated by both the innerreflector and the outer reflector, the power density is effectivelysuppressed. Under the condition set to keep the total power of thereactor core constant, the output is increased at the downstream side ofthe flow path owing to the suppressed amount of output at the upstreamside of the flow path. Thus, the nuclear reactor according to the stillanother aspect of the present invention is advantageous in that thepower distribution in the reactor core is effectively equalized in theflow direction of the fuel pebbles.

The nuclear reactor (pebble bed reactor) according to the still anotheraspect of the present invention is advantageous in comparison with thenuclear reactor according to the other aspects in that the powerdistribution in the reactor core is more effectively equalized in theflow direction of the fuel pebbles.

In the nuclear reactor (pebble bed reactor) according to still anotheraspect of the present invention, since the criticality of the reactorcore is adjusted by the enrichment of the fuel pebble, the output can bemaintained.

In the nuclear reactor (block reactor) according to still another aspectof the present invention, the volume ratio of the graphite to themoderator which has a smaller moderating power than the graphite in eachportion of the reflector is determined so as to optimize the powerdistribution in the reactor core. Therefore, the power density of thereactor core at each position in the coolant flow path is adjusted.Thus, the nuclear reactor according to the still another aspect of thepresent invention is advantageous in that the power distribution of thereactor core can be adjusted, and that the power distribution in thereactor core can be equalized and/or optimized accordingly.

In the nuclear reactor (block reactor) according to still another aspectof the present invention, the volume fraction of the moderator which hasa smaller moderating power than graphite is set high in the downstreamside of the coolant flow path (lower side of the reactor core) tosuppress the maximum power density and/or the maximum fuel temperature.Thus, the nuclear reactor according to the still another aspect of thepresent invention is advantageous in that the power distribution in thereactor core can be adjusted and that the power distribution in thereactor core in the flow direction of the coolant can be effectivelyoptimized so that the power density decreases towards the downstreamside.

In the nuclear reactor according to the present invention, thecomposition of the reflector (in particular the volume fraction of themoderator that has a smaller moderating power than graphite) isdetermined so as to optimize the power distribution in the reactor core.Therefore, the degree of moderation of neutrons at each position in theflow path of the fuel bodies is adjusted. Thus, the nuclear reactoraccording to the present invention is advantageous in that the powerdistribution in the reactor core can be adjusted, and that the powerdistribution in the reactor core can be optimized accordingly.

In the nuclear reactor according to the present invention, since thevolume ratio of the graphite to the moderator which has a smallermoderating power than the graphite in each portion of the reflector isdetermined so as to optimize the power distribution in the reactor core,the power density of the reactor core at each position in the coolantflow path is adjusted. Thus, the nuclear reactor according to thepresent invention is advantageous in that the power distribution in thereactor core can be adjusted, and that the power distribution of thereactor core can be equalized and/or optimized accordingly.

As can be seen from the foregoing, the nuclear reactor according to thepresent invention is useful in that the power distribution in thereactor core can be equalized and/or optimized without the need ofrepetitious reloading of the fuel pebbles (in the pebble bed reactorcore) and without the need of preparation of fuel in various differenturanium enrichment levels (in the block-type reactor core).

Although the invention has been described with respect to a specificembodiment for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art that fairly fall within the basic teaching herein setforth.

1. A nuclear reactor of a Block-type High Temperature Gas Reactorcomprising: a reactor core; a fuel block arranged in the reactor core,the fuel block having a coolant flow path through which a coolant flowsto cool the fuel block; and a reflector arranged inside the reactor coreand sectioned into plural parts along a flow direction of the coolant,the reflector containing graphite and a moderator having a smallermoderating power than the graphite, wherein a volume ratio of thegraphite to the moderator having a smaller moderating power than thegraphite in each part of the reflector is determined based on a powerdistribution in the reactor core in the flow direction of the coolant,and a volume fraction of the moderator having the smaller moderatingpower than the graphite in each part of the reflector is determined sothat the volume fraction increases from an upstream side to a downstreamside of the coolant flow path.
 2. The nuclear reactor according to claim1, wherein the reflector is sectioned into five parts along the flowdirection of the coolant, so that the volume fraction of the moderatorhaving the smaller moderating power than the graphite increases by 25%in each part from the upper side toward the lower side of the reactorcore.